Ethylene and propylene are important fundamental raw materials for petrochemical products and are mainly produced by thermal-cracking of paraffin-based hydrocarbons such as natural gases, naphthas and gas oils at a high temperature of at least 800° C. in the presence of steam. In general, gaseous raw materials include ethane, propane, butane and a mixture thereof and liquid raw materials include naphthas, kerosenes, gas oils and a mixture thereof. Besides olefins as major product, a large amount of byproduct including hydrogen, methane, acetylene, carbon monoxide, carbon dioxide or the like is produced. A diluent, which is mixed with the hydrocarbon and then put into a heating furnace, is supplied to build the reaction condition for obtaining a desired product. The diluent lowers a partial pressure of the hydrocarbon reactant to reduce the amount of the byproduct such as hydrogen or methane and restrict an amount of coke produced in the tube type furnace to the minimum amount. Steam is preferred as the diluent.
The tube type furnace includes a radiation part for heating the reaction mixture to thermally crack the reaction mixture by radiant heat transfer resulted from burning of fuel such as gas oil or natural gas in a burner and a convection part for receiving supplied reaction raw materials and preheating the reaction raw materials by convective heat transfer resulted from the burned gas exhausted from the radiation part. At this time, the reaction raw materials make a stream flowing rapidly in a circular tube arranged in parallel or in a coil shape from the convection part to the radiation part of the furnace. Heat generated by the burning of the fuel in the burner placed in the radiation part of the furnace is transferred to the reaction stream through the circular tube, thus resulting in generation of the thermal-cracking reaction. After that, the cracked gas is rapidly cooled in a heat exchanger.
In general, a temperature of the hydrocarbon stream at an outlet of the convection part of the furnace is set in the range of 450˜650° C., a temperature (COT; coil outlet temperature) of the hydrocarbon stream at an outlet of the radiation part of the furnace is set in the range of 750˜950° C. and a residence time of the gaseous hydrocarbon in the tube of the radiation part is set in the range of about 0.01˜1.5 seconds, although they are set differently depending upon the kind of the hydrocarbon supplied to the hydrocarbon thermal-cracking furnace. Also, although a diameter and a length of the tube may vary as the design of the thermal-cracking furnace, the tube has generally an inside diameter of 25˜200 mm and a length of 8˜100 m. Such tube can resist high temperature and is made of high temperature thermostable metal containing high content of nickel, iron and chromium. Example of the mainly used thermostable metal includes Incoloy 800, Inconel 600, HK-40, Cr—Mo steel alloy, 304 SS and 316 SS. However, the component such as nickel or iron is well known as a catalyst which causes formation of coke. Since the hydrocarbon thermal-cracking process should be performed in a very short time and a sufficient amount of heat for thermal-cracking should be supplied in order not to cause generation of undesirable side reaction through the thermal-cracking in the radiation part, the hydrocarbon thermal-cracking process consumes very much energy.
During the thermal-cracking of the hydrocarbon, carbon or similar deposits is generated and deposited to inside wall of the tube, which is an important limitation factor in an operation of the thermal-cracking furnace. Such deposits reduce an effective sectional area to cause an increase in a differential pressure between the circular tubes of the furnace and heat exchanger, thereby reducing yield of the light olefins. The deposits of carbon component act as a good insulator, which may prevents the heat transfer from the furnace to the reaction stream and thus increases more the fuel consumption. Also, uniformity of the heat transfer may be reduced, and therefore the production of the deposits may be accelerated and properties of the tube material may be deteriorated. Accordingly, when a temperature of the tube is increased to at least a predetermined temperature during continuous operation of the furnace at a reference performance, the operation of the furnace should be temporarily paused to perform removal of the carbon deposits. The removal of the carbon component deposits in the furnace and heat exchanger is largely divided into a physical cleaning and a decoking that removes the carbon component deposits by burning the carbon component deposits using steam/air. This carbon deposit removal takes a week to three months depending upon a deposition speed of the carbon component deposits in the thermal-cracking furnace and the heat exchanger. Although this carbon deposit removal is necessary to maintain the apparatus performance, it is advantageous in aspects of productivity and economy that the interval of the carbon deposit removal is as long as possible. There is therefore a need for a thermal-cracking apparatus with reduced fuel consumption by improved heat transfer efficiency, reduced production of the coke deposited within the tube and a long decoking interval.
Meanwhile, in order to increase yield of ethylene and propylene in a steam cracking of hydrocarbon, it is required higher conversion rate of hydrocarbon or higher selectivity of olefin. However, the steam cracking alone has limitations in increasing the hydrocarbon conversion rate or the olefin selectivity. In this regard, there have been suggested various methods capable of increasing the olefin yield.
As a method for increasing the yield of ethylene and propylene in the hydrocarbon steam cracking, there has been suggested a catalytic steam cracking. U.S. Pat. No. 3,644,557 discloses use of a catalyst including magnesium oxide and zirconium oxide; U.S. Pat. No. 3,969,542 discloses use of a catalyst consisting essentially of calcium aluminate; U.S. Pat. No. 4,111,793 discloses use of a zirconium oxide-supported manganese oxide catalyst; European Patent Publication No. 0212320 A2 discloses use of a magnesium oxide-supported iron catalyst; and U.S. Pat. No. 5,600,051 discloses use of a catalyst including barium oxide, alumina, and silica. Also, WO2004/105939 discloses use of a catalyst including potassium magnesium phosphate, silica and alumina. However, use of these catalysts is known to increase the olefin yield by action of the catalyst material as a heating medium in the hydrocarbon steam cracking and thus has a problem that an increase in the yield of olefins is insignificant as compared to use of an inactive carrier.
Russian Patent No. 1,011,236 discloses a boron oxide-grafted potassium vanadate catalyst supported on an alumina carrier. However, in use of the alkaline metal oxide or potassium vanadate catalyst, an increase in the olefin yield due to the catalyst is small and inevitable losses are also generated at high temperature for the hydrocarbon thermal-cracking. In other words, the catalyst may be present in a liquid phase in a hot reactor due to its low melting point and catalyst components may therefore be dissipated by volatilization with time due to fast flow of reaction gases.
U.S. Pat. No. 7,026,263 discloses use of a hybrid catalyst including molybdenum oxide, alumina, silica, silicalite and zirconium oxide. Such catalyst has an advantage that a reaction can be performed at a low reaction temperature, but it is difficult to apply the catalyst directly or indirectly into existing process since the catalyst is used at a very low hydrocarbon feed rate. Also, thermal stability of the catalyst is significantly decreased at a reaction temperature of at least 700˜800° C. and loss of the catalytic activity is thus resulted.
Further, since existing thermal-cracking process is performed at high reaction temperature and high hydrocarbon linear velocity and is accompanied with a large amount of coke generation, it is necessary to burn the generated coke at a high temperature. In order that the catalyst can be used for a long time in such severe operation condition, the catalyst should be stable against thermal/physical deformation. The abovementioned prior arts have a problem that they are weak to the thermal/physical deformation or stability thereof is not verified.
Accordingly, when considering an economic aspect of the steam cracking of hydrocarbon or in order to avoid a process complexity, there is required a catalyst with more significantly increased light olefin yield than that in the inactive carrier and excellent thermal/mechanical stability at a high temperature.